1. Field of the Invention
The invention generally relates to power line communications, and more particularly, to avoiding magnetic saturation in a power line inductive coupler.
2. Description of the Related Art
In a power line communication system, power frequency is typically in a range of 50–60 Hertz and a data communication signal frequency is typically in a range of 1–50 MHz. An inductive coupler for coupling modulated data onto and off of power lines includes a magnetic core through which pass the power line and a secondary winding. This is described, for example, in patent application Ser. No. 09/752,705, filed Dec. 28, 2000, patent application Ser. No. 10/082,063, filed Feb. 25, 2002, and provisional patent application No. 60/364,321, filed Mar. 14, 2002, which are incorporated herein by reference. One limitation to the usage of such couplers on high current primary and secondary wires is the limited ability of the magnetic cores to withstand large magnetization currents without magnetic saturation.
Saturation degrades performance in two ways. First, saturation reduces the magnetization inductance, which increases the shunt admittance loading on the data signal. Second, the instantaneous magnitude of power line current rises from zero to a peak value twice each cycle on ac power lines, and the data signal is amplitude modulated at twice the power line frequency and harmonics of this double frequency. So if the modulation depth exceeds some particular value, this modulation causes data errors.
One way to minimize core saturation is to introduce one or more air gaps in the magnetic circuit of the core, thus increasing magnetic resistance and permitting higher currents to flow before saturation is encountered. But the inductances of the primary and secondary windings fall in proportion to the total magnetic resistance, so a wide air gap would imply an impracticably large mass of core would be needed to have a minimum acceptable magnetization inductance. Increasing the number of turns is also impractical, because the coupler is usually installed on large diameter power lines without interrupting the power circuit.